I have added a 'comments' field in a pardot form. This field by default maps to 'pardot comments' in SF. If someone fills the form and gives comment, this value is shown in 'pardot comment' field in SF, however if the visitor filling the form is already there in our record, his 'comment' field value is not shown.
Pardot by default will not show fields in a form when they have already been filled out.
When editing form fields there is the option checkbox on the "Advanced" tab to select "Always display even if previously completed"
Use for fields for which the prospect can submit the same value more than once. If this option isn’t enabled, the field doesn’t display again after a prospect has completed it.